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What to know about calories and body fat

In relation to food and the body, calories are units of energy that allow the body to work. Food provides this energy, some of which the body stores and some of which it uses. As the body breaks down food, it releases calories as energy.

Max Wishnofsky first propagated the concept that there are approximately 3,500 calories in a pound (lb) of body fat.

Put simply, to lose 1 lb of body fat per week, people will need to have a deficit of around 500 calories per day. They can achieve this by consuming roughly 500 calories fewer than they are currently, by burning an extra 500 calories per day with exercise, or a combination of the two.

If the body takes in too many calories or burns too few, weight gain occurs. This is because the body stores calories it does not use as body fat. Organs including the brain, heart, lungs, liver, and kidneys account for roughly 80% of total daily energy use.

Recent research calls this rule into question, concluding that it overestimates someone's weight loss potential. The rule does not take into account dynamic changes in metabolism, hunger, and satiety levels as weight loss occurs.

The National Institutes of Health (NIH) have developed a new, more accurate rule-of-thumb: Every 10 calorie decrease per day leads to an eventual 1 lb loss. Only time will tell how long that weight loss takes, so patience and consistency is key.

How fat works

Having too much or too little body fat can cause health problems.

Body fat, or adipose tissue, consists of adipocytes.

These are fat cells, and they occur alongside other types of cells and proteins. Fat cells contain lipids, including cholesterol and triglycerides.

Adipose tissue stores energy for the body to use and protects the organs. It also releases hormones that control many functions in the body, such as insulin sensitivity and appetite.

People with more body fat may experience something called leptin resistance, in which the body is less sensitive to the satiety hormone leptin. This, in turn, drives up hunger and food intake, making weight maintenance harder over time.

There are two types of adipose tissue: white and brown. Brown adipose tissue is more metabolically active. It burns more calories and helps manage weight, insulin sensitivity, and overall health to a greater extent than white adipose tissue.

If people have excess body fat, it is most often due to the fact that their white adipose tissue has expanded.

How calories work

Research estimates that muscle burns calories at a rate of 10–15 calories per kilogram (kcal/kg) per day. This amounts to 4.5–7 kcal/lb per day.

Muscle accounts for roughly 20% of total energy expenditure each day. For people with 20% body fat, body fat accounts for 5% energy expenditure.

So, people with more muscle tissue have a higher metabolic rate. This means that they burn more calories and are able to maintain their body weight more easily.

How do we calculate calories?

Various apps are available that can help with tracking calories intake.

Calories in food do not amount to precisely the same measurement inside the body as outside the body.

That said, scientists measure the amount of caloric energy food contains by using a device called a bomb calorimeter.

By burning the food in this device, scientists can measure the heat released to find out the number of calories in the food.

This provides a figure for the total potential energy of food. However, this is not a true reflection of how the body will use the energy from food. The body cannot always use all of the calories that people consume.

Calorie losses can happen due to:

urinating

incomplete digestion of food

having an ineffective metabolism

How people burn calories will depend on their metabolism, digestion, and overall health and fitness levels.

A calorimetry machine can show how many calories people burn when resting. This is called their basal metabolic rate. The machine measures carbon dioxide, which is the waste product from food the body burns as energy. Other accurate methods of estimating calorie expenditure use air or water displacement technology.

Using these tools, people can work out an accurate number for the calories they burn during rest and activity or exercise. It is important recheck this figure every 3–6 months to understand how the body composition and calorie needs change over time.

People can use calorie-counting tools and fitness trackers to build a rough idea of how many calories they are consuming and how many they are burning. However, this will not always be completely accurate.

Are different types of fat higher or lower in calories?

These are rough estimates, however, since specific foods affect insulin demands, gut bacteria, and digestion and absorption differently. All of these factors affect the calories per gram of food and an individual's metabolic rate.

Certain fats are more healthful than others. Consuming too many trans and saturated fats can raise the levels of harmful cholesterol in the body and increase the risk of heart disease.

Monounsaturated and polyunsaturated fats are good for the body. Some good sources of these fats include oily fish, nuts, seeds, and vegetables.

The body needs a certain amount of healthful fat to function properly. Research suggests that although there is no single dietary macronutrient plan that will work for everybody due to individual needs, most health experts recommend the following amount of carbohydrates, proteins, and fats for a balanced diet:

Healthy fats: 20–35% of calories

Protein: 15–20% of calories

Complex carbohydrates: The remaining percentage of calories

How many calories should I eat to lose weight?

Nutrient dense foods are important when eating to lose weight.

If people want to lose 1–2 lbs per week, they will need a calorie deficit of 500–1,000 calories per day.

People can lose weight by reducing their calorie intake, but it is also important that the calories people are eating come from healthful, nutrient-dense foods.

A simple way to reduce calorie intake and eat a balanced, healthful diet is to swap certain foods for lower-calorie options.

For example, eating a sandwich with extra vegetables and salad instead of extra slices of cheese and ham can reduce calorie intake by up to 154 calories.

Consuming low-calorie foods can still keep people feeling full if they contain plenty of water and fiber.

Below are some examples of lower-calorie food swaps that people can make:

using cooking oils that contain less saturated fat, such as olive, avocado, sunflower, or peanut oil

using a spray oil for cooking in order to use less of it

if using canned foods, such as beans, tomatoes, or fish, choosing options that have no added salt or rinsing them under water to remove some of the salt

if eating canned fruit, choosing items that have juice rather than syrup

People can also combine a reduction in calorie intake with exercise to burn off stored calories in the body.

Summary

Past research has shown that there are roughly 3,500 calories in 1 lb of body fat. This has led many people to believe that if they want to lose 1 lb of body weight, they just need a reduction of 3,500 calories.

For example, this would mean losing 500 calories each day to lose 1 lb of body fat in a week. Though weight loss and human metabolism are much more complex than this basic calculation, it is clear that a deficit in calories is necessary for weight loss.

However, the length of time it takes to lose a specific amount of weight varies among people.

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